Data storage, whether fixed storage, such as solid state memory and direct access storage devices (DASD), or removable storage, such as magnetic media (diskette and tape) and optical media, is available in many capacities and, therefore, in many prices. Generally, a customer will purchase and install enough storage capacity to meet the customer's immediate and near term growth needs. When the customer's storage needs begin to exceed the existing capacity, the customer purchases and installs additional capacity. When the storage is DASD or other fixed media, expansion involves purchasing and installing additional DASD units. When the storage is removable media, expansion can be as simple as purchasing additional blank media.
In some situations, however, it may be more convenient (and ultimately less expensive) for the customer or the supplier to physically install storage capacity to meet the customer's current and expected longer term needs. If the customer cannot afford, or does not wish to pay for, the full capacity, the supplier may install the full capacity but charge the customer only for the smaller amount needed. Later, when the customer needs or can afford the additional capacity, the customer can purchase from the supplier the right to use the additional previously installed capacity.
It will be appreciated that, even if the customer agrees not to use the additional originally installed but unpurchased capacity, it may be difficult for the supplier to prevent the customer from doing so, thereby losing potential revenue. In the case of an automated data storage and retrieval library system for tape or optical media, it may be possible for the supplier to install empty storage racks and later install individual storage cells in the racks.